A hypothesis proposed in the early 1960's was that new ocean floor is created when two plates move away from one another at mid-ocean ridges. Basically basalt it being pushed up from the earth and cooled by the ocean water. So oceanic crust is slowly always moving in a cycle.
Sea floor spreading is a cycle. It starts from magma being pushed up into the ocean (large amounts of basalt). The magma gets cooled and left on the ocean floor. As more and more basaltic magma comes up the older rock gets pushed across the ocean floor. Eventually the floor slides under a continent and back into the earth to repeat the cycle.
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